This invention relates to an integrated digital/analog converter, with an input signal which accepts digital data lines which are to be converted to an analog signal output corresponding to the magnitude of the input signal.
Every bit of the digital input signal is designed to control a specific electronic switch which handles a specific constant current. The constant current delivered by a specific constant current source is binarily weighted in such a manner that the value of the constant currents conducted through the individual switches correspond in each case to the value of the corresponding bit in the digital signal to be converted. Each of these switches--which are identical to one another--consists of a differential amplifier, in which identical transistors are arranged together, with their current input terminals connected to a respective constant current source. The switches are controlled at their control terminals by a respective bit of the digital data lines in such a manner that for a control bit corresponding to the logic "1" state, the constant current generated by the respective constant current source is switched for all these switches to a common signal output that is related to the analog signal to be generated, while for a control bit corresponding to the logic "0" state, the constant current is applied to another common signal output or to the base of the respective constant current source.
In an embodiment of a conventional digital/analog converter of this kind, the weighting of individual constant currents is made possible by the corresponding measurement of the constant current source that generates them and it is realized by a specific transistor, in particular, by symmetrical current switching. In a familiar embodiment of this kind, the electronic switch which is controlled by the LSB (least significant bit) of the digital data signals is attached to a constant current source which consists of a transistor with an emitter, in which the emitter is connected to a reference potential and the base to a control potential that is common to all the constant current sources. To the switch controlled by the bit with the next lower value there is then connected a transistor with two emitters, which delivers a current that is doubled with respect to the first current source. The bit with the next lower value, to which is assigned a constant current with a quadruple value has a transistor with four emitters as a constant current source, etc. It is, of course, also possible to connect a number of identical transistors that corresponds to the required weight factor in parallel with one another; they will deliver the weighted current at their common output. This method is of particular interest when dealing with a circuit of the D/A converter that consists of MOS field effect transistors of the enhancement type, particularly when the transistors are identical.
On the other hand, it is also possible to use with each individual switch a specific constant current source, which, in and of itself, delivers the same current at the other constant current sources. The weighting is then accomplished by an appropriate resistance network, which connected the constant current delivered by the individual constant current sources through the converter that is assigned to each one with the common output in various ways and by this means achieve the desired weighting.
In summary, it can be stated that in the digital/analog converters corresponding to the definition given above, weighted currents are switched to or from the output thus generating the desired analog signal, according to the respective input data (that is, the digital signal that is to be converted). In this case, however, experience has shown that as the result of non-simultaneous activation of the switches current peaks or current drops are unintentionally produced which can sometimes degenerate the output analog signal. This phenomenon is known as the GLITCH effect. It can be reduced by a balanced layout of the integrated circuits that make up the D/A converter but it cannot be completely eliminated.